rMBP 2015 doesn't recognise Dell P2715Q over DisplayPort

I recently purchased both a Dell P2715Q 27" 4K monitor and a 13" 2015 rMBP. The monitor is not recognized when I plug the laptop into the monitor via the monitor's provided DP cable (into either Thunderbolt port). When I plug in the laptop using the Monitor's HDMI cable, it shows up and can render at 4K, but only at 30Hz. I also tried plugging the laptop into another display with HDMI and that one also worked fine. Unfortunately, I don't have access to another display that has a DP port. My research prior to purchasing the laptop and monitor suggested that I should be able to connect them easy-peasy and get 4K at 60hz using either MST or SST signalling. However, I am very new to Macs, so my question is whether or not I need to do something special to get this to work? A Thunderbolt to DP cable perhaps? The monitor also has a miniDP port as well, do I have to use that? Did I somehow end up with a broken DP cable?

Any ideas?

So oddly enough, 10.10.3 didn't seem to make any difference, but the supplemental update that came up yesterday (14D130a) on Software Update (http://9to5mac.com/2015/04/16/apple-releases-os-x-10-10-3-supplemental-update-to-fix-startup-issue/) fixed it right up. Now when I connect the display via Thunderbolt, it comes up at the native 4K resolution at 60hz running in SST mode. Thanks everybody for your help!